The present disclosure relates to articles incorporating nanopillars or nanowires, such as terahertz metamaterials, and methods of manufacture. More particularly, some aspects relates to high quality-factor terahertz metamaterials, such as micro-scale split ring resonators, as well as a resonant behavior induced by a displacement current that can be utilized with the terahertz metamaterials. Other aspects relate to methods for manufacturing such terahertz metamaterials as well as other nanopillar or nanowire articles.
Terahertz metamaterials (THz MMs) are good candidates as sensors for the detection of chemicals and biomaterials, temperature, strain, alignment, and position. THz MMs can also be used as frequency-agile devices by adding a dielectric material around the MMs. The sensing resolution and frequency selectivity of the MMs depends on their quality factors (Q-factors) because high Q-factors mean the MMs have sharp resonant responses, allowing detection of small frequency shifts induced by substances around the MMs. Even though THz MMs show great promise for sensing and tunable devices, their relatively low Q-factors (typically below 20 of single-ring resonator MMs) compared to micro- and nanoscale mechanical resonators (typically between 104 and 107) impose limitation on their sensitivity.
One of the approaches to increase the Q-factor of MMs is to reduce the energy losses of MMs and substrates by optimizing the material properties and structures of the MMs. There are typically three main energy loss mechanisms: Ohmic loss of MMs, dielectric loss of the substrate, and radiation loss of MMs. The most common method to increase Q-factor of MMs without changing material properties is to design asymmetric split resonators (ASRs) by breaking the symmetry of the MMs. The asymmetric design reduces the radiation loss of the resonator and can increase the Q-factor up to 30. Another method uses coupling between MMs in a super unit to excite both odd and even modes of the MMs. This approach can improve the Q-factor by a factor of 5 compared to typical film-based MMs. However, the Q-factor of THz MMs needs to be further enhanced (10 to 20 times) to meet the requirement of ultra-sensitive sensors.
Another factor that measures the sensitivity of MM sensors is how much the resonant frequency shift in the transmission spectrum when permittivities of the adjacent medium change. Modern detection techniques require sensors to have the ability to detect a very small quantity of substances, even single molecules. However, it is extremely difficult to achieve such a high sensitivity using typical film-based MM sensors because the response to changes of substance, in the form of small resonant frequency changes, can be hard to detect, especially when the volume or concentration of the substance around the MMs is not high enough. In order to develop sensors that can detect minute concentration of substances, large resonant frequency change in response to the change of the substance around the MMs is one of the key requirements.